Use of "dont"

Question

Write "The girl I'm thinking of is Isabelle." : La fille ________ je pense est Isabelle.à quidontà laquelle

I have read most of the discussion about the use of "dont", and I don't understand why it's wrong in this question. Can anyone explain it - simply, if possible. auquelà quoi

Accepted Answer

Bryce, 
because in this context,  it is  ' penser à qqn ', not ' penser de ...'.  - Je pense à Isabelle.
As a relative pronoun, dont can only 'replace' the complement when it would be introduced by the preposition ' de '. 
When the preposition introducing the complement being replaced would be " à ", you need to use ' à qui ' or ' auquel (or its variants), as the lesson is covering. 
( With any preposition other than de, the basic form is ' preposition qui/lequel ', with the usual contractions when the preposition is ' à '. ) 
See ' à noter ' in the Lawless link on 'dont' below.
 https://www.lawlessfrench.com/grammar/penser-lesson/
 https://www.lawlessfrench.com/grammar/relative-pronoun-dont/
 https://progress.lawlessfrench.com/revision/grammar/a-qui-auquel-a-laquelle-to-whom-what-which-relative-pronouns